The present invention relates to a method for adjusting an electron beam apparatus to minimize its aberration due to astigmatism and field curvature of focus, and particularly, a method of calibrating a current to be fed to correction coils.
Electron beam exposers are widely used for mask making or pattern exposure in semiconductor production, especially in integrated circuits (IC) or large scale integrated (LSI) circuits. As the rate of integration increases, such as for very large scale integrated (VLSI) circuits, the pattern to be exposed on a chip becomes both fine and large. For example, the chip size may be more than 10.times.10 mm.sup.2, but the line width to be exposed may be less than one .mu.m. In such a large chip having such fine patterns, aberration and distortion of the electron beam becomes a limiting factor in pattern exposure.
In order to overcome such difficulties, the pattern of a chip is subdivided into plurality of sub-patterns, and the sub-patterns are exposed one by one, shifting the exposure area from sub-pattern to sub-pattern. The main pattern and sub-pattern of the chip are called, in the art, a main field and a sub-field, therefore, these terms will be used hereinafter. Even for such subdivided patterns (fields) the limiting factor of the resolution is the aberration and distortion of the electron beam. Further, it is undesirable to divide the field into a large number of sub-fields, because this increases the time necessary to expose the entire field and increases the errors. Therefore, it is desirable to decrease the aberration and distortion as much as possible for a large scanning area.
Errors of images exposed by electron beams are divided into aberration and distortion. Aberration includes astigmatism, field curvature of focus, coma aberration and transverse chromatic aberration. Coma and transverse chromatic aberrations are considered intrinsic in an electron beam apparatus because of the design of the focusing system, and are difficult to correct. On the other hand, astigmatism and field curvature of focus can be corrected, together with distortion. Therefore, most modern electron beam exposers are provided with correction coils to correct such errors.
Another bottleneck in electron beam exposure is processing speed, because more than one million exposures are required to expose the entire surface of a wafer. Therefore, the exposure is controlled by computer and the process is generally automated. But the corrections for astigmatism and field curvature of electron beam are still performed manually. So the correction is done only at the center or at some predetermined spots of the sub-pattern, and the whole sub-pattern is exposed using only the correction factors for the predetermined spots. So far as the inventors know, there have been very few attempts to perform the calibrations at several points in the pattern with the aid of a computer. Even when such attempts have been made, the calibration for the portions of the deflection field other than the calibrated points is carried out using the appropriate interpolation or mean value of those correction factors. By prior art methods, therefore, it is impossible to attain the best possible resolution of an electron beam over the entire surface of the chip to be exposed.
To clarify the advantages of the present invention over the prior art, a prior art method for electron beam exposure will be described briefly. FIG. 1 illustrates an ordinary method for exposing an IC substrate with a electron beam. Explanations will be made for the parts which relate to the present invention. As shown in FIG. 1(a), the IC wafer 2 is mounted on a table 1 which slides in the X and Y directions, called an XY-table. An electron beam 8 is focused on the surface of the slice 2 by an electron beam lens system. Using a main deflector 7 and a sub-deflector 9, the electron beam 8 is scanned over the substrate to expose a chip with a predetermined pattern. When the exposure of a chip is completed, the XY-table 1 slides to bring the main field of the next chip to the exposure area.
FIG. 1(b) illustrates a plan view of an IC wafer 2 placed on the XY-table 1. Wafer 2 has a square shape as shown in the side view of FIG. 1(a); however, wafers 2 usually have a circular form. A patterning area 3 of the wafer 2 is divided into a plurality of main fields 4, as shown in FIG. 1(b). Each main field corresponds to one chip of an IC, and a plurality of chips are fabricated on a wafer. An exposure is performed for each main frame (chip), and when the exposure is completed, the XY-table slides one pitch of the main field.
Each main field 4 is further divided into a plurality of sub-fields 5 as shown in FIG. 1(c). The electron beam 8 is deflected by main deflector 7 to each of the sub-fields and each sub-field is scanned by the electron beam 8 using the sub-deflector 9 to expose patterns 6. When the exposure for a sub-field is completed, the beam is deflected to the next sub-field. In such a manner, when all the sub-fields, i.e., the entire chip, is exposed, the XY-table slides to bring the next main frame (chip) into the exposed area.
Furthermore, FIG. 1(a) also illustrates, schematically, a part of electron beam exposer which is related to the present invention. Recent electron beam exposers are provided with means to correct the astigmatism and field curvature of focus, shown by coils 10 and 11. Coil 10 corrects the astigmatism, and is a so-called "stigmator coil." Coil 11 corrects the field curvature of focus. The calibration of the beam is performed by adjusting the current running through these coils. The present invention concerns a method of operating such correction means, and does not relate to the structure of such means. Thus, the details of these coils and the structure of the electron beam exposer are omitted for the sake of simplicity.
As illustrated in FIG. 2, astigmatism is an aberration. FIG. 2 is a plan view showing a beam spot on various parts of scanning area, and emphasizing the deformation of spot. Although the beam is focused in a circular point at the center of the scanning area, it deforms to an oval when the beam is deflected. The deformation of the spot can be corrected by varying the current of coils 10x and 10y (the coils will be described later with respect to FIG. 4.). Since the deformation varies at each point of the scanning area, the correction current should also be varied at each point of the scan.
Abberration due to the field curavature of focus is illustrated in FIG. 3. FIG. 3(a) shows, schematically, a side view of an electron beam. Generally, an electron beam is focused on a spherical surface 12, whose radius is related to the focal length of the electron beam lens. If the electron beam 8 is focused on the center C of a flat main field 13 and if a deflected beam 8' is directed to a point P, the beam spot will blur, as illustrated in FIG. 3a. Additionally, a square pattern, for example, will be deformed as shown in FIG. 3(b). The aberration shown in FIG. 3(a) can be corrected by adjusting the current of the coil 11. The curvature of the pattern shown in FIG. 3(b) is not aberration, but deformation, so it can be corrected by adjusting subdeflector 9 when correcting other deformations. The present invention relates to the correction of aberration of an electron beam spot due to astigmatism (FIG. 2) and the aberration due to the field curvature of focus (FIG. 3(a)). It will be obvious that this kind of aberration must be corrected at every point in the scanning area in order to attain the maximum resolution over the entire field of scanning.
Astigmatism and field curvature of focus are theoretically different phenomena and are independent of each other. But, in practice, if one of them is corrected by a correction means (coils), the other is affected. Thus, there is a correlation between them for the correction means, making the calibration more difficult.
FIG. 4 is a block diagram illustrating a conventional system for calibrating the astigmatism and field curvature of focus. There may be many types of systems for this purpose, so FIG. 4 shows a typical system. The system comprises a computer 31 which is a central processing unit (CPU). The CPU 31 controls not only the calibration, but it also aids the operator in obtaining the correction factors. The CPU 31 also controls the entire electron beam exposure system. For example, control of the XY-table, deflection and scanning of electron beam and turning the electron beam on and off to expose the pattern are all controlled by CPU 31. These functions, however, are omitted from FIG. 4 for the sake of simplicity, and will not be described herein.
In the example of FIG. 4, pattern data which describes the position and size of patterns is stored on a magnetic tape 32, and correction data which describes the correction factors of each beam position is stored on a magnetic tape 33. Before starting the exposure, the CPU 31 reads out and stores such data, respectively, in a pattern data buffer memory 34 and a correction data memory 35.
Correction data stored in the memory 35 includes:
stig x, the correction data for astigmatism in X direction;
stig y, the correction data for astigmatism in Y direction;
focus F, the correction data for aberration due to the field curvature of focus; and
other correctio data such as the correction of shrinkage or expansion of sub-fields (denoted x and y in the figure), and distortions (denoted dx and dy in the figure). This data is used for correcting the electron beam under the control of the CPU 31, but only the use of stig x, stig y and focus F will be described herein.
When the exposure begins, the CPU assigns the sub-filed and pattern position. Then the correction data stig x, stig y and F corresponding to the sub-field and pattern position, the position to where the electron beam is deflected, are read from the correction data memory 35 and sent to a register 36, where they are stored, respectively, as values of the current to be supplied to the stigmator coils 10x, 10y and focus correction coil 11 (the current values are denoted Ix, Iy and IF, respectively). The respective data signals are sent to an adder (ADD) 38 and then to a digital analog converter (DAC) 39 associated with each of the coils 10x, 11y and 11, where they are converted to analog signals, amplified by amplifiers 40 and fed to coils 10x, 10y and 11, respectively. At the same time, the CPU 31 controls the electron beam exposer to deflect the beam 8 to the position determined by the same pattern data. So the beam is corrected by the correction data stored in the magnetic tape 33.
The system of FIG. 4 is provided with a second register 37 which is used for computer aided measurements. Operation of the second register 37 and the adders 38 will be explained later with reference to an embodiment of the present invention.
The problem is how to develop the correction data--in other words, how to obtain the data stored in the correction data memory 35 in FIG. 4. The present invention is related to a method of obtaining the data of stig x, stig y and focus F. As mentioned before, these three types of correction data are interrelated, and thus it is very difficult to obtain the optimum point.
Correction data can be obtained from a measurement of sharpness of the electron beam and various methods may be designed for measuring the sharpness of an electron beam. One example which is often used will be explained with reference to FIGS. 4 through 7. The measurement of the sharpness of the beam can be carried on manually or with an aid of a computer. At first, it will be explained how the sharpness of the beam spot is determined and how it is measured.
FIG. 5 is a schematical diagram showing part of the electron beam exposer which concerns the present invention. In FIG. 5, reference numeral 53 designates a detector which collects the scattered electron beam from the surface of the substrate 50. The detector 53 may be an electron collector for scattered or secondary electrons or an electron collector for the portion of the beam transmitted through the substrate. Recent electron beam exposers are provided with such a detector and the explanation below will be made with respect to a system having a scattered electron detector. The substrate 50 has a test mark on its surface, as shown in FIG. 7. The test mark is made from tantalum or gold film and is approximately 5-50 .mu.m square.
The electron beam 8 is scanned over the test mark, and the scattered electrons are collected by the collector 53 and amplified by an amplifier 54. FIG. 6(a) illustrates this situation with a side view of the electron beam 8. When the electron beam 8 crosses the edge of the test mark 57, the current of the scattered electrons varies as shown in FIG. 6(b). Since the electron beam 8 has a spot size (not shown in the figure), the change in the measured current varies with a slope as the beam crosses the edge of the test mark, as shown in FIG. 6(b). This slope corresponds to the spot size, that is, the sharpness of the electron beam 8 at the position of the test mark. If the current is differentiated over time for the position x or y, by a differential circuit 55, the output of the differential circuit 55 is in the form of a pulse, as shown in FIG. 6(c). The size of this pulse also corresponds to the sharpness of the electron beam at the position of the test mark. Such measurement can be performed manually or with the aid of the CPU 31 shown in FIG. 5. In this case, the output of the differential circuit 55 is fed to an analog to digital converter (ADC) 56, and then sent back to the CPU 31 for a calculation of the correction factor.
FIG. 5 shows, schematically, how the astigmatism and field curvature of focus are corrected in a conventional electron beam exposer. Stigmator coils 10X and 10Y correct the astigmatism in X and Y directions. The stigmator coils 10X and 10Y are composed of four coils (not shown), connected in series, to construct an electron lens. The axes of the electron lens 10X and 10Y are orthogonal to the axis of the electron beam 8, and are aligned to cross each other with an angle of 45.degree.. By adjusting the current running through coils 10X and 10Y, it is possible to correct the deformation of beam spots due to the astigmatism. A third coil 11 composes another lens for correcting the aberration of focus due to the field curavature of focus by adjusting the current running through the coil.
The correction data is obtained as follows. First, the electron beam is deflected in the X and Y directions to an edge of a test mark (two types of test marks are shown in FIGS. 7(a) and (b)), which is moved to a position where correction data is desired. The sharpness of the beam is measured in the manner described above, manually or with the aid of a CPU. For example, at first the sharpness p.sub.x for the X direction is measured, keeping I.sub.y, the correction current for the stigmator 10Y, and I.sub.F, the correction current for the coil 11, constant. Next, I.sub.x, the correction current for the stigmator 10X, is varied, causing a corresponding variation in the sharpness p.sub.x, and the current I.sub.x for the maximum value of the p.sub.x is determined. Generally, this value is considered as the correction factor for the coil 10X. Similarly, the correction factors for coils 10Y and 11 are determined by keeping the correction currents for other coils constant.
As has been mentioned before, however, thre are correlations between these corrections. So if the maximum value of p.sub.x is measured while I.sub.y and I.sub.f are fixed, then, for example, p.sub.x will vary when I.sub.y or I.sub.F are varied. In order to avoid this problem, the measurements are repeated several times, setting the correction current for other coils to that of the previous measurement. Thus, the calibration for each coil is optimized asymptotically by repeating the measurements, but this method requires an extended period of time.
To obtain the calibration factors, therefore, the measurement is usually performed for only one point per sub-frame, or for several points per main frame. The calibration for other parts of the chip is determined from such data or from a mean or interpolation of the data. Accordingly, it has been impossible to obtain optimum focusing conditions over the entire surface of the chip, and it has been impossible to attain the maximum resolution of the electron beam exposer. Moreover, though it is theoretically possible to calibrate the focusing at all points over the chip, it is necessary to renew the correction data occasionally, for instance once a week or once a day before the beginning of the exposure process, in order to attain the maximum resolution because the stability of such apparatus is very critical. However, such calibration has been impossible and impractical for prior art electron beam exposers.